Fabrication and characterization of sulfonated silicaNafion composite membrane for proton exchange membrane fuel cells to improve proton conductivity under low relative humidity condition

Abstract

Polymer exchange membrane fuel cells (PEMFC) have been received considerable attention due to high energy efficiency and environmental compatibility [1]. The commercialized membrane such as Nafion is used as a core part to PEFCs. The perfluoro sulfonic acid (PFSA) membrane has good physicochemical stability as well as excellent proton conductivity [2]. Due to these good characteristics, PEMFCs can be used for transportation system, portable devices, and even unmanned aerial vehicle (UAV). To apply PEMFCs into UAV, the main components of PEMFCs related with membrane electrode assembly (MEA) have been received great attention, especially electrolyte membrane. Under fully humid state at 70 to 80 °C, the Nafion based membranes show outstanding fuel cell performance. However, the Nafion based membrane show low proton conductivity under low relative humidity (RH) which means the MEA which include Nafion membrane as a part of main components exhibit low fuel cell performance due to low proton conductivity [3]. In order to successfully mount and operate PEMFCs on a UAV, the fuel cell performance should be improved under low RH condition. In addition, since the PEMFCs to be installed in UAV should be reduced in weight, it is inevitably required to use oxygen in the air and the operating temperature is low. To overcome this technical barriers, alternative electrolyte membranes with high proton conductivity at low RH condition have to be developed. Over the last decades, researchers developed alternative electrolyte membranes by using hygroscopic inorganic fillers such as titania, zirconia, and silica which retain water molecules and narrow the ionic channels in the membrane which means that these facilitate the proton conduction when it inserted in Nafion membrane as a filler [4]. Even though, the hygroscopic fillers can help facile proton conduction, the number of sulfonate groups in the Nafion interacted with these fillers, exhibiting low proton conductivity. In this study, a sulfonated silica (SS) is used as a filler for Nafion membrane to enhance proton conductivity under low RH. The SS/Nafion composite membrane was prepared by solution casing method. The SS/Nafion composite membrane exhibit better proton conductivity than recast Nafion membrane which was fabricated for comparison. Other characteristics such as water uptake, IEC, mechanical property, and detailed value of proton conductivity as function of RH will be discussed.